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Vol. 16, Issue 4, 1839-1849, April 2005

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A Rab Requirement Is Not Bypassed in SLY1-20 Suppression

Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755

Submitted August 21, 2004; Revised January 6, 2005; Accepted January 18, 2005
Monitoring Editor: Suzanne Pfeffer

The Rab GTPase Ypt1p and the large homodimer Uso1p are both required for tethering endoplasmic reticulum-derived vesicles to early Golgi compartments in yeast. Loss-of-function ypt1 and uso1 mutations are suppressed by SLY1-20, a dominant allele that encodes the Sed5p-associated protein, Sly1p. Here, we investigate the mechanism of SLY1-20 suppression. In wild-type strains, Ypt1p can be coimmunoprecipitated with Uso1p; however, in a ypt1/SLY1-20 strain, which lacks this complex, membrane binding of Uso1p was reduced. In spite of Ypt1p depletion, Uso1p-dependent vesicle tethering was not bypassed under the ypt1/SLY1-20 condition. Moreover, tethering and fusion assays with ypt1/SLY1-20 membranes remained sensitive to Rab GDP dissociation inhibitor. These results indicate that an alternative Rab protein satisfies the Ypt1p requirement in Uso1p-dependent tethering when SLY1-20 is expressed. Further genetic and biochemical tests revealed that a related Rab protein, Ypt6, might substitute for Ypt1p in ypt1/SLY1-20 cells. Additional experimentation to address the mechanism of SLY1-20 suppression in a cog2 [sec35] strain indicated that the Cog2p subunit of the conserved oligomeric Golgi complex is either functionally redundant or is not directly required for anterograde transport to the Golgi complex.

Address correspondence to: Charles Barlowe (barlowe{at}dartmouth.edu).

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